1. Field
Embodiments of the invention relate to a method of reducing audible noise emanating from magnetic cores based on amorphous magnetic materials such as transformer cores. Further embodiments relate to magnetic cores having reduced audible noise.
2. Description of the Related Art
Iron-based amorphous alloy ribbon exhibits excellent soft magnetic properties including low magnetic core loss under AC excitation, finding its application in energy efficient magnetic devices such as transformers, motors, generators, energy management devices including pulse power generators and magnetic sensors. In these devices, amorphous ferromagnetic materials with high saturation inductions and low magnetic core loss are preferred. Although these features have been achieved in Fe-based amorphous alloys, their magnetostriction values tend to be somewhat higher than those of conventional crystalline Fe—Si alloys. Magnetostriction is one of the intrinsic properties of magnetic materials and is characterized by dimensional changes when the materials are magnetized from their remanent states. When a magnetic material expands along the direction of magnetization, the phenomenon is termed positive-magnetostrictive. When a magnetic material shrinks upon magnetization, the effect is called negative-magnetostrictive. In either case, the material vibrates mechanically under an AC excitation. Thus, when the material is used in a magnetic core that is under an AC excitation, the core emanates sound. One example is the familiar hum from electrical distribution transformers. Due to the on-going increase of population density in residential areas, the transformer noise is becoming an issue. Since the magnetostriction of a material is determined by its chemical composition and atomic or crystal structure, the sound level from a magnetic core is controlled by the design and fabrication of the core based on a given core material. Thus, the design and fabrication of a magnetic core based on amorphous magnetic materials must be optimized for its lowest sound levels, which is an aspect according to an embodiment of the present invention.
The amorphous Fe-based alloys are cast into ribbon forms due to the need for rapid solidification of molten alloys. The commercially available amorphous magnetic ribbon has a thickness ranging from about 15 μm to about 50 μm. When the relatively thin ribbon is wound to form a large-sized magnetic core, the side of the core must be mechanically reinforced to maintain its mechanical integrity. This is the case when the core is used as a distribution transformer core that has a physical cut in order that transformer electrical conductor windings can be inserted into the core. For example, U.S. Pat. No. 4,734,975 (hereinafter '975 patent) describes a method of coating the sides of a transformer core by using epoxy resin to strengthen the core mechanically. This method is currently used in a number of transformers based on amorphous alloy ribbon. During curing of the resin, however, mechanical stress is introduced on the sides of the core due to the thermal expansion coefficient difference between the core material and the resin, which increases the core's magnetic loss and exciting power. These increases in turn result in increased transformer's audible noise. Thus the effect must be mitigated, which is another aspect according to an embodiment of the invention. An additional aspect of the invention is to search for environmental-friendly core reinforcement materials. Currently, used polymer coating materials, such as epoxy resin, adhere strongly to the metallic magnetic cores but generate hazardous gases when the cores are remelted during recycling, which needs to be mitigated.